Why does the male Mexican molly fish have an 'extravagant moustache-like structure' on its top lip? German and American scientists may have discovered the answer: Because female fish dig it. For oral sex.

The molly is a freshwater fish found in Mexico. Some of the male members of the species have growths above their mouths that vaguely resemble mustaches, if you're into anthropomorphizing fish. No one has ever been able to figure out why the fish have 'staches. Were they growing them "ironically"? Trying to look older? Just big Tom Selleck fans?

[Scientists] caught from the wild a selection of male and female Mexican mollies, measuring the length of the moustache on those males found to be growing one.

The research team then conducted a series of experiments, placing male and female fish into tanks, and measuring how long females spent in the company of males sporting moustaches of various lengths, or none at all.

They also measured how female fish responded to videos of different males.

The results were clear: on experiments involving over 100 fish, females consistently preferred males with moustaches.

There you have it! Chicks dig mustaches, you know, biologically. Well, fish chicks, in any case.

But the most important bit of scientific potential here, if you are the kind of person who is interested in making sex jokes, is the possible reason for the female molly's mustache preference:

Although the scientists only tested the visual attractiveness of the moustache, they strongly suspect it also has a tactile function.

"This is based on the general observation that males will touch the female's genital region with their mouth prior to mating," Prof Schlupp told the BBC.

This behaviour is known as 'nipping' and is being investigated further by the scientists.